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August 17, 2017 | last updated August 17, 2017 12:18 pm

Innovative roof — Maine's first — installed at Portland's Canal Plaza

Courtesy / Canal 5, Michael Boucher Landscape Architecture
An architect's drawing shows what the Canal Plaza complex in downtown Portland that's now under construction by Consigli Construction will look like when it's completed this fall.

Construction of a freestanding concrete building at the Canal Plaza Complex in downtown Portland will incorporate an innovative roof system that's new to Maine.

Consigli Construction in Portland this week installed a Cobiax Technology system that lightens the load of the roof slab and allows the slab to span the distance between exterior walls, as opposed to a conventional slab roof that would require support columns.

Casting of the roof began at 4 a.m. on Aug. 14. The choice of the Cobiax system was made in response to certain challenges of the project, said Matthew Tonello, who runs Consigli's Portland office.

The building shell is expected to be complete by the beginning of October. It is expected to house a cafe or retail. When it was proposed last year, developer East Brown Cow Management Inc. said it would cost $5 million to transform the plaza, the Portland Press Herald reported at the time. The area being developed sits between the Canal Plaza buildings, which date to the 1970s.

Canal Zero, as the project is known, was designed to make the plaza more inviting.

The structural engineering design of the roof structure was performed by Portland-based Becker Structural Engineers.

Other local contractors include South Portland-based Sebago Technics, Freeport-based Michael Boucher Landscape Architecture and Portland-based Canal 5 Studio, which is based at 1 Canal Plaza. Canal 5 and Michael Boucher Landscape Architecture collaborated on the master plan.

A system integrating plastic and steel

One challenge for Consigli was the unusual shape of the building, which Tonello characterized as the shape of a guitar pick — a combination of six different radii. In addition, the interior was to be a free-and-clear span, without any supporting columns. Finally, the building was to be constructed from concrete, as an aesthetic choice.

"This was outside of what we typically see for steel beam and columns or for wood-framed walls and floors," said Tonello.

The system consists of fixed reinforcement steel elements with already-integrated "void formers" made from rigid plastic. The system essentially resembles parallel lengths of steel cages assembled into modules and lined with beach balls. For this installation, the modules came in 10-foot lengths.

Cobiax has been developing the systems since 1997 for projects worldwide. This is the first time this type of system has been used in Maine, says Canal Zero Project Manager Travis Kirby.

The building is 1,300 square feet.

"Having a free and open area under the roof was important to the client to maximize occupancy and the area for potential tenants to utilize," said Kirby.

An unusual shape with unusual challenges

Given the structure of an odd shape of the structure at Canal Zero, Consigli deployed three innovative techniques to solve the problem. The Cobiax system was combined with a product called self-consolidating concrete, also known as self-compacting concrete, that's highly flowable and spreads itself into place by means of its own weight. Then they deployed a "post-tensioning" system similar to using reinforcement steel except for the tensioning step: When a concrete slab is stressed by the post-tensioning method, it means the steel is being tensioned and the concrete is being compressed and held together more tightly, according to concretenetwork.com.

For a roof of 1,350 square feet, typically, Kirby said, a concrete slab that size would weigh 405,000 pounds. The Cobiax system displaced 77,000 pounds of that weight.

The unique system costs more upfront, said Tonello, but is worth it in the long run.

"It takes concrete out of the slab that isn't actually being used to do the work of holding itself up," he said.

Redevelopment of the rest of the plaza will include installation of 3,000 square feet of granite veneer on existing walls and 18,000 square feet of granite pavers.

Time-lapse video showing Consigli Construction's innovative Canal Zero project in Portland
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